Display screen or portion thereof with a graphical user interface



The FIGURE is a front view of a display screen or portion thereof with a graphical user interface showing the new design.

The broken lines illustrating an electronic device form no part of the claimed design. The broken lines illustrating the display screen and the remaining portions of the graphical user interface form no parts of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a graphical user interface, as shown and described. 